Powder coating compositions for use in painting surfaces have become increasingly desirable in recent years for various reasons including those relating to ecology, health and safety. More particularly, such powder coating compositions have become desirable because they eliminate the solvents utilized in liquid paint systems such as are currently in wide use. The application and drying or curing of such liquid paint compositions, of course, requires that the solvent vehicle be volatilized, thus resulting in solvent escaping into the atmosphere and creating health and safety hazards as well as undesirable pollution problems. Powder coating compositions, on the other hand, are curable by heat in a manner such that little, if any, volatile material is given off to the surrounding environment.
While numerous powder paint compositions have been proposed, coatings formed from such compositions share various problems including low luster, poor color development, poor color matching and poor shading. The problems associated with such coatings are at least in part a result of the processes by which these powders are prepared. Prior art manufacturing methods for providing powder coating materials include ball milling, Z-blade mixing and extrusion.
Ball milling is the simplest of these three methods. In the manufacture of epoxy based powder coatings, for example, all of the ingredients such as granulated epoxy resin, hardener or curing agent, pigments and additives are loaded into a ceramic lined ball mill. Ceramic grinding media of various size and shape are generally used to grind the materials for ten to fifteen hours to produce a blend. Other types of ball milling can be used to produce a similar blend in a shorter time. This method, however, does not achieve a good dispersion of pigments and other additives, thus resulting in powders which cure to form coatings of exceptionally low gloss and opacity. In addition, such a process does not allow for satisfactory color and shade matching of the coating.
In Z-blade mixing resin is first heated in a Z-blade mixer to at least its melting point and when molten all other ingredients except curing agent are added slowly. In general, dispersion requires about 6 hours after which the temperature of the Z-blade mixer is reduced and curing agent is added. As soon as the curing agent has been adequately mixed, the melt is cooled, pulverized and classified. The compositions formed by this process generally produce coatings having inadequate pigment dispersion and gloss requirements for top coat uses such as for automobiles. As was the case with the ball milling process it is also difficult to obtain adequate color and shade matching with this process. Also since the process is a batch process, the mill has to be thoroughly cleaned after each run because of the build-up of heat sensitive material.
In the extrusion method, the resin being used is blended with all other ingredients in a high speed mixer and the mixture is then fed into a heated extruder. The extrudate is cooled, pulverized and classified. Although the coatings produced by this method are generally of better quality than those formed from powders made by the above processes, they still, like the others, suffer from lack of flexibility in providing color and shade matching. A still further disadvantage of this process stems from the fact that pigment is dispersed by extrusion, thus requiring slower extrusion rates and possible premature cross-linking due to longer dwell times at elevated temperatures. This premature cross-linking results in a powder which forms coatings having surface imperfections and reduced gloss.
A copending application entitled Method of Preparing Powder Paint Compositions-I and filed concurrently herewith teaches a method of preparing powder paints which obviates many of the problems associated with prior art techniques such as discussed above. The method of that application comprises: introducing a liquid paint composition including at least a cross-linkable copolymer and uniformly dispersed pigment into an evaporating zone adapted to heat said liquid paint so as to vaporize the inert solvent contained therein; passing the liquid paint into a separating zone; causing said solvent vapor to be removed; removing the non-volatile components of said paint in a molten state; cooling said non-volatile components; and pulverizing to form a powder. Although powder paints prepared according to that method form coatings superior to those prepared by the prior art techniques, the process is subject to certain problems. In particular, when the copolymer being employed requires the presence of a cross-linking agent for curing, that process either requires that the cross-linker be mixed with the liquid composition and processed through the evaporating and separating zones or blended with the copolymer powder, subsequent to processing, in a high speed mixer or the like. When the first alternative is followed, i.e., including cross-linking agents before processing, the composition may have a tendency to prematurely cross-link during processing. The degree to which this occurs will, of course, depend upon various factors such as the copolymer used, the particular cross-linking agent employed, the necessary dwell time within the evaporating and separating zones, the necessary temperature within the evaporating and separating zones, etc. In any event, when such premature crosslinking, i.e., that occuring during processing, occurs to any appreciable extent, the final cured coatings formed from the powders exhibit surface imperfections ("orange peel") and decreased gloss which may make the coating unsuitable for many top coat applications.
When the cross-linker is merely blended with the uniformly pigmented powder in accordance with the second alternative, it is difficult to obtain adequate distribution of the cross-linker. As a result such powder paint compositions may cure unevenly and exhibit diminished gloss and reduced exterior durability.
In view of the desirability of producing powder paint compositions useful as top or finish coats and further in view of the above-discussed shortcomings of the prior art or alternative techniques for producing powders including cross-linking agents it is the object of this invention to provide a method of preparing powder paints whereby a cross-linking agent is thoroughly and uniformly mixed with the copolymer to be cross-linked without a danger of premature cross-linking.